Use the ShellExecute function in that button click event.
syntax:
HINSTANCE ShellExecute(
_In_opt_ HWND hwnd,
_In_opt_ LPCTSTR lpOperation,
_In_ LPCTSTR lpFile,
_In_opt_ LPCTSTR lpParameters,
_In_opt_ LPCTSTR lpDirectory,
_In_ INT nShowCmd
);
for eg:
ShellExecute(0,0,"www.google.com",0,0,SW_SHOWNORMAL);
when you click on that button the default browser automatically opens with that URL.
All The Best.